In view of portability, the electronic book reader employs a storage cell as a power source. Generally, the storage cell needs to be charged from an electric outlet (AC power source) every one to two weeks. This is burdensome for a user. Therefore, a solar cell is used to attempt to extend the charging interval.
For example, there is a commercially available product configured so that an amorphous silicon (a-Si) solar cell and another storage cell are attached to a cover of the electronic book reader, the other storage cell is charged with the a-Si solar cell, and upon completion of the charging, the storage cell of the electronic book reader is charged via a USB cable from the other storage cell. However, the product has a problem in that a user is required carry the other storage cell in addition to the storage cell in the electronic book reader, resulting in an increased weight and cost.
Further, in a display system described in PTL 1, power from a solar cell is charged to a secondary cell, circuitry is operated with the power from the secondary cell, and prescribed information is displayed in an electronic paper in accordance with an operation status of the circuitry. Moreover, in another display system described in the PTL 1, a solar cell and a cell (a primary cell or a secondary cell) are each connected to the circuitry via a capacitor circuit, and the circuitry is electrically connected to the electronic paper. When the power from the solar cell and the power from the cell are used to drive the circuitry, a prescribed display is performed in a display unit of the electronic paper.
Further, in a power source apparatus described in PTL 2, power from a solar cell is charged to a lead storage cell and a capacitor module. The power source apparatus acquires solar irradiance from a sensor and determines whether or not the solar irradiance is stable on the basis of a difference in solar irradiance. Upon the solar irradiance being stable, the power source apparatus charges the power to a lead storage cell which takes time to charge. On the other hand, upon the solar irradiance being unstable, the power source apparatus switches the charging target to the capacitor module and charges the capacitor module. The capacitor module is constituted by one or more capacitors having a relatively small electrostatic capacity. The power source apparatus acquires a charged current extractable from the solar cell, calculates a charging time per capacitor, sequentially switches a plurality of capacitors in accordance with the charging time, and charges the capacitors.
In an electronic paper described in PTL 3, whether or not the power supplied by a solar cell is equal to or greater than a voltage value capable of controlling the electronic paper is detected. Upon the power being less than the voltage value, the electronic paper is controlled with the power from the storage cell. On the other hand, upon the power being equal to or greater than the voltage value, the electronic paper is controlled with the power supplied by the solar cell, and after completion of an image update process, redundant power is charged to the storage cell.